1. Field of the Invention
The present invention relates to a high frequency circuit including a transistor.
2. Description of the Background Art
Conventionally, various high frequency circuits such as amplifiers using active devices such as bipolar transistors or field effect transmitters have been used. FIG. 18 is a circuit diagram of a conventional emitter grounded wide-band amplifier using a bipolar transistor.
The amplifier shown in FIG. 18 comprises a bipolar transistor (hereinafter abbreviated as a transistor) 200, resistors 11, 12, 13, 14, and 15, and capacitors 16, 17, and 18. The transistor 200 has its base connected to a node N11. The node N11 is connected to an input terminal NI through the capacitor 16. The resistor 11 is connected between a power supply terminal receiving a power supply voltage VCC and the node N11, and the resistor 12 is connected between the node N11 and a grounding terminal.
The transistor 200 has its collector connected to the power supply terminal through the resistor 13, and is connected to an output terminal NO through the capacitor 17. The transistor 200 has its emitter connected to the grounding terminal through the resistor 14, and is connected to the grounding terminal through the resistor 15 and the capacitor 18. An input signal is fed to the input terminal NI, and an amplified output signal is outputted from the output terminal NO.
A DC bias point of the transistor 200 shown in FIG. 18 is determined by a resistance value R1 of the resistor 11, a resistance value R2 of the resistor 12, a resistance value RC of the resistor 13, a resistance value RE of the resistor 14, and the power supply voltage VCC. As the DC bias point of the transistor 200, a base voltage VB, an emitter voltage VE, a collector voltage VC, and a collector current IC are considered. First, the base voltage VB is expressed by the following equation:VB=R2·VCC/(R1+R2)  (1)
The emitter voltage VE is expressed by the following equation:VE=VB−VBE  (2)
where VBE is a voltage between the base and the emitter. The voltage VBE between the base and the emitter is generally constant, for example, approximately 0.6 to 0.7 V. Further, an emitter current IE is expressed by the following equation:IE=VE/RE  (3)
A base current IB becomes a value significantly smaller than the collector current IC and the emitter current IE, and is generally approximately one-hundredth the collector current IC and the emitter current IE. Therefore, IE=IC in an approximate manner. Consequently, the following equation (4) holds:VC=VCC−IC·RC  (4)
In the amplifier shown in FIG. 18, if it is assumed that VCC=15 [V], VBE=0.6 [V], R1=100 [kΩ], R2=12 [kΩ], RE=1 [kΩ], and RC=10 [kΩ], the DC bias point of the transistor 200 is VB=1.6 [V], VE=1.0 [V], IE=IC=1.0 [mA], and VC=5 [V].
Then consider voltage gain in the amplifier shown in FIG. 18. If the capacitance value CE of the capacitor 18 inserted on the side of the emitter of the transistor 200 is large, and its impedance is sufficiently small, voltage gain AV in a low frequency region of the amplifier is expressed by the following equation:AV=RC/RX  (5)
where RX is a composite resistance value of the resistors 14 and 15 which are connected in parallel, and is expressed by the following equation:RX=RE·REE/(RE+REE)
The voltage gain AV in the low frequency region of the amplifier is determined by the ratio of the resistance value RC of the resistor 13 to the composite resistance value RX of the resistors 14 and 15 which are connected in parallel, and does not depend on the DC bias point. In the above-mentioned bias conditions, the voltage gain AV in direct current is 10. If the resistance value REE of the resistor 15 is taken as 1 kΩ, for example, however, the voltage gain AV in the low frequency region is 20.
In order to prevent the voltage gain AV in the low frequency region from being reduced in the above-mentioned amplifier, however, the capacitor 18 having a large capacitance value CE must be used. In a case where the amplifier shown in FIG. 18 is constituted by an integrated circuit, therefore, when a capacitor having an MIM (Metal-Insulator-Metal) structure having a relatively large capacitance value per unit area is used as the capacitor 18, the occupied area thereof is increased. Accordingly, the amplifier cannot be miniaturized.
In the actual amplifier, a mirror effect is produced. Accordingly, the gain thereof in a high frequency region is reduced. Here, the mirror effect is such a phenomenon that when a capacitor is connected to an input terminal and an output terminal of the amplifier, a case where the capacitance value of the capacitor is small is equivalent to a case where a capacitor having a large capacitance value is connected, as viewed from the input side.
Generally in the bipolar transistor, an internal parasitic capacitance exists between a base and a collector. The capacitance value of the internal parasitic capacitance is increased due to the mirror effect. An internal resistance in the transistor, for example, a base parasitic resistance and the internal parasitic capacitance equivalently increased due to the mirror effect form a low-pass filter. The low-pass filter exerts adverse effects such as the reduction of the gain in the high frequency region.
The effect of the mirror effect in the amplifier shown in FIG. 18 will be described. FIG. 19 is an equivalent circuit diagram of the amplifier in a case where the transistor is represented by a hybrid π-type equivalent circuit. FIG. 20 is an equivalent circuit diagram of the amplifier in a case where consideration is given to the mirror effect.
In FIGS. 19 and 20, rb and rπ indicate internal parasitic resistances in the transistor 200, and Cπ and CC indicate internal parasitic capacitances in the transistor 200. The internal parasitic capacitance CC between the base and the collector is a capacitance value which is multiplexed by (AV+1) due to the mirror effect. Consequently, the frequency characteristics AV(f) of the voltage gain in the amplifier is expressed by the following equation in an approximate manner:AV(f)=AV/(1+jωCTrT)  (6)
where f is a frequency, and ω is an angular frequency. Further, CT and rT in the foregoing equation (6) are expressed by the following equation:CT=Cπ+CC(1+gmRL)=Cπ+(1+AV)·CCrT=rb·rπ/(rb+rπ)
where gm is a mutual inductance of the transistor 200. Further, the frequency fC which is lowered to 3 dB (=1/√{square root over (2)}), as compared with the voltage gain AV in the low frequency region, is expressed by the following equation (7):fC=1/(2πCTrT)  (7)
The frequency characteristics of the wide-band amplifier is thus limited by the frequency fC. When the foregoing equation (6) is rewritten using the foregoing equation (7), the following equation is obtained:|AV(f)|=AV/{1+(f/fC)2}  (8)
From the foregoing equation (8), the voltage gain is reduced when the frequency is raised. When the reduction in the gain in the high frequency region is compensated for by adjusting the capacitance value CE of the capacitor 18, the gain in the low frequency region is reduced.
On the other hand, a negative feedback amplifier comprises an amplification portion and a feedback portion. Generally, a resistor having no frequency dependency is used for the feedback portion, and the amplification portion is composed of a transistor or the like and has frequency dependency. The gain G of the negative feedback amplifier is expressed by the following equation:G=A/(1−A·β)
where A is open-loop gain (the gain of the amplification portion itself), and β is a feedback factor. When the open-loop gain A is reduced because the frequency is raised, the gain G of the whole negative feedback amplifier is reduced. In the wide-band amplifier or the like, therefore, the upper-limit frequency is limited.
In order to solve this, a method of decreasing a feedback amount as the frequency is increased by constituting the feedback portion by a capacitance, an inductance, and a resistance has been known. When such an inductive or capacitive element is used, however, the phase of a feedback signal varies depending on the frequency. Accordingly, the feedback portion enters a positive feedback state at a predetermined frequency, thereby causing problems. For example, the stability of the amplifier is degraded.
An object of the present invention is to provide a high frequency circuit capable of controlling characteristics in a high frequency region with a small occupied area and without degrading characteristics in a low frequency region.